1. Field of the Invention
The present invention relates to an optical waveguide capable of converting a mode field diameter at a desired portion thereof, such as a mode field diameter conversion fiber and a planar light waveguide device.
2. Related Background Art
A conventional beam expanding fiber is disclosed in the reference titled as "Beam Expanding Fiber Using Thermal Diffusion of the Dopant" in Journal of Lightwave Technology. Vol. 8, No. 8 August 1990. The beam expanding fiber of the above reference has a core in which Ge is induced and the induced Ge is thermally diffused so that a spot size of the propagating mode, which corresponds to "mode-field diameter of the optical fiber", is partially expanded.
By increasing the mode field diameter at the end of the optical fiber as shown in the above reference, it is possible to insert an optical device between the fibers with the expanded mode fields without significant loss increase.
The above conventional fiber, however, includes a problem that a long time heating is needed to increase the mode field diameter.